improve display of subject headings in default skin

[Galen Charlton]

Currently the BibTemplate code in rdetail/rdetail_summary.xml for display of subject headings only includes the 650. It should be expanded to:

* include all standard subject added entry fields (or more broadly, all 6XX)
* exclude control subfields (those whose subfield label is a digit) from display
* only using the double-dash separator before subfields $x, $y, $z, and $v. The display text of other subfields should be separated from one another only by a space.

[Ben Ostrowsky (sylvar)]

Here's a patch to address this.

[Dan Scott (denials)]

Thanks Ben! I've applied a variation of this patch to trunk (see the commit log comment in http://svn.open-ils.org/trac/ILS/changeset/18456 for a description of the differences) - how far back do we want to port this?